Shorted Aquastat Relay

Del

Hi everyone-



  Was on another no heat call today.  Beckett AFG burner and Weil McLain boiler and a honeywell single zone aquastat relay.  Customer said the circuit breaker to the furnace had tripped.  Checked out the aquastat and I had power at L1 and L2 coming in and the contactor would close when a call for heat was initiated. Turns out the 'stat was burned out on the neutral side on both the circulator and burner circuits so I replaced it and all is well.  My question has to do with some basic circuitry.  When I was testing the 'stat with my meter, I had one lead on the return pipe(ground) and I checked the hot on both the burner and circulator and I got 120V on each, as well as 120V at the primary control (both the incoming from the aquastat and the outgoing to the blower/transformer) When I moved the lead I had on the return pipe to the L2 neutral on the stat, I also had 120V but when I put it on the neutrals on the burner and circulator circuits I got nothing.  So, thats when I decided the neutrals were shorted out and sure enough there was a big black mark on the back of the board.  How come I still was getting 120V on the burner/circ. circuits when I had the ground on the return pipe?  I'm thinking I understand the problem, but I'd like an explanation from someone who has been doing this longer than I have!



Thanks

 Del

JStar

Sounds like...

...the neutral connection between L2 and the burner/pump circuits was open.



When you had your leads on any hot leg and a known ground (the return pipe), you completed the circuit and got a reading. But when you placed your leads on a hot leg and one of the system neutrals, which was basically just an open wire, you weren't completing any electrical circuit to ground. It was as if your lead was out in the open air.